Control of brake systems is an important aspect of automotive functionality. Two common controlled brake system functions are ABS and TCS. TCS events occur when traction is lost while the vehicle is accelerating. ABS events occur when traction is lost while the vehicle is braking.
During anti-lock braking (ABS) events, it is important to keep a rear axle within two-wheel drive (2WD) systems, a non-engaged axle within front-wheel drive (FWD) systems, or a non-powered axle within all-wheel-drive (AWD) systems at or near vehicle speed in order to not degrade vehicle stability. If the speed of the rear/non-engaged/non-powered axle is reduced too much, the lateral capability of the tires connected to the axle and the roadway is reduced.
Under normal circumstances, rear/non-engaged/non-powered wheel speeds will recover to vehicle speed after brake pressure is released. Unfortunately, if the coefficient of friction of the road surface is low the wheels may not recover quickly.
Various methods exist for forcing the wheels to recover more quickly. One such method for rear wheel drive vehicles equipped with electronic throttle control (ETC) includes increasing engine torque slightly to force the rear wheel speeds to increase. Unfortunately, increasing engine torque too much may result in rear wheel spin and a corresponding reduction of vehicle stability.
During a TCS event it is important to regulate the driven wheels speeds to a level that maximizes vehicle acceleration while maintaining vehicle stability and steerablity. The appropriate level of the driven wheel speed is slightly faster than the true reference velocity of the vehicle. Thus is it important that an accurate estimate of the true reference velocity of the vehicle be known. On an AWD vehicle, in which both axles are powered, the calculation of a vehicle reference velocity is complicated when all 4 of the wheels are spinning excessively. Various methods exist for calculating vehicle reference velocity in this situation. Once such method uses a longitudinal accelerometer to obtain an estimate of the vehicle reference velocity. Unfortunately, this method is subject to inaccuracies due to road grades and sensor signal disturbances on rough roads.
It would be desirable, therefore, to provide a system that would overcome these and other disadvantages.